1. Field of the Invention
This invention relates to improved recovery of hydrocarbons from a subterranean formation by a combustion process (Fireflooding). In one aspect, the invention relates to an improved combustion process wherein the improvement involves prevention and/or mitigation of channeling of combustion front movement from a lower permeable hydrocarbon-bearing stratum through a semi-permeable stratum and through an upper permeable hydrocarbon-bearing stratum between an injection well and a production well wherein the improvement comprises injecting an aqueous fluid selectively to the upper hydrocarbon-bearing stratum and injecting an oxygen-containing gas selectively into the lower hydrocarbon-bearing stratum.
2. Brief Description of the Prior Art
A great need exists for increased production of hydrocarbons to meet increasing demands in the face of rapidly depleting reserves. One of the more promising approaches to providing for this need involves enhanced recovery methods. Thermal recovery methods, in particular, in situ combustion methods, provide one means for recovering vast reserves of heavy petroleum deposits including tar sands and other reservoirs containing high viscosity materials which are not economically recoverable by other means.
U.S. Pat. No. 3,208,519; 2,994,375; 3,406,755; 3,159,215; 3,170,515; 3,024,841; 3,196,945; and 3,171,479 are exemplary disclosures of meritorious processes for recovery of heavy hydrocarbons by thermal methods, in particular in situ combustion methods (Fireflooding), which are particularly related to the instant invention.
Thus, it is known to recover hydrocarbons from a hydrocarbon-bearing subterranean formation, in particular a heavy oil reservoir or tar sand, by penetrating the formation with a production well and an injection well, igniting the hydrocarbons in the deposit, injecting air to cause burning of a portion of the hydrocarbons in situ, and recovering hydrocarbons which are reduced in viscosity by the heat generated by the burning. Processes involving forward combustion wherein an oxygen-containing gas is injected into an injection well causing forward burning in the direction of a production well are known. Also known are reverse combustion processes wherein combustion is initiated in a production well with oxygen-containing gas injection from an injection well and movement of the firefront from the production to the injection well and production of hydrocarbons from the production well. It is also known to enhance the effectiveness of such fireflood processes by introduction of water into proximity with the burning zone. It is further known (U.S. Pat. No. 3,208,519) to inject water near the upper portion of a hydrocarbon-bearing stratum wherein both water and air are injected through an injection well to move a firefront from an injection well to a production well. It is also known to inject water through an injection well into an upper hydrocarbon-bearing stratum while air is injected into a lower permeable hydrocarbon-bearing stratum through an injection well to move a combustion front through the lower stratum wherein both strata are separted by an impervious shale stratum (U.S. Pat. No. 3,159,215).
Advantageous and valuable though such processes are, such do not completely fulfill the need for an efficient method of recovering hydrocarbons from hydrocarbon-bearing reservoirs wherein an upper permeable hydrocarbon-bearing stratum overlies a lower hydrocarbon-bearing permeable stratum and is separated therefrom by a semi-permeable stratum which is substantially permeable to oxygen but not substantially permeable to water. Our improvement invention provides a substantial advance in recovering hydrocarbons from hydrocarbon-bearing strata in reservoirs of the type described.